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Diffstat (limited to 'doc/rados/operations/placement-groups.rst')
| -rw-r--r-- | doc/rados/operations/placement-groups.rst | 32 |
1 files changed, 22 insertions, 10 deletions
diff --git a/doc/rados/operations/placement-groups.rst b/doc/rados/operations/placement-groups.rst index dda4a0177..93ab1f0c0 100644 --- a/doc/rados/operations/placement-groups.rst +++ b/doc/rados/operations/placement-groups.rst @@ -4,6 +4,21 @@ Placement Groups ================== +Placement groups (PGs) are subsets of each logical Ceph pool. Placement groups +perform the function of placing objects (as a group) into OSDs. Ceph manages +data internally at placement-group granularity: this scales better than would +managing individual RADOS objects. A cluster that has a larger number of +placement groups (for example, 150 per OSD) is better balanced than an +otherwise identical cluster with a smaller number of placement groups. + +Ceph’s internal RADOS objects are each mapped to a specific placement group, +and each placement group belongs to exactly one Ceph pool. + +See Sage Weil's blog post `New in Nautilus: PG merging and autotuning +<https://ceph.io/en/news/blog/2019/new-in-nautilus-pg-merging-and-autotuning/>`_ +for more information about the relationship of placement groups to pools and to +objects. + .. _pg-autoscaler: Autoscaling placement groups @@ -131,11 +146,11 @@ The output will resemble the following:: if a ``pg_num`` change is in progress, the current number of PGs that the pool is working towards. -- **NEW PG_NUM** (if present) is the value that the system is recommending the - ``pg_num`` of the pool to be changed to. It is always a power of 2, and it is - present only if the recommended value varies from the current value by more - than the default factor of ``3``. To adjust this factor (in the following - example, it is changed to ``2``), run the following command: +- **NEW PG_NUM** (if present) is the value that the system recommends that the + ``pg_num`` of the pool should be. It is always a power of two, and it + is present only if the recommended value varies from the current value by + more than the default factor of ``3``. To adjust this multiple (in the + following example, it is changed to ``2``), run the following command: .. prompt:: bash # @@ -168,7 +183,6 @@ The output will resemble the following:: .. prompt:: bash # ceph osd pool set .mgr crush_rule replicated-ssd - ceph osd pool set pool 1 crush_rule to replicated-ssd This intervention will result in a small amount of backfill, but typically this traffic completes quickly. @@ -626,15 +640,14 @@ pools, each with 512 PGs on 10 OSDs, the OSDs will have to handle ~50,000 PGs each. This cluster will require significantly more resources and significantly more time for peering. -For determining the optimal number of PGs per OSD, we recommend the `PGCalc`_ -tool. - .. _setting the number of placement groups: Setting the Number of PGs ========================= +:ref:`Placement Group Link <pgcalc>` + Setting the initial number of PGs in a pool must be done at the time you create the pool. See `Create a Pool`_ for details. @@ -894,4 +907,3 @@ about it entirely (if it is too new to have a previous version). To mark the .. _Create a Pool: ../pools#createpool .. _Mapping PGs to OSDs: ../../../architecture#mapping-pgs-to-osds -.. _pgcalc: https://old.ceph.com/pgcalc/ |